Gold plating



United States Patent 3,214,292 GOLD PLATING Gwynne I. Edson, Bethlehem, Pa., assignor to Western Electric Company, Incorporated, New York, N.Y., a corporation of New York No Drawing. Filed Sept. 12, 1962, Ser. No. 223,239 6 Claims. (Cl. 117-227) The present invention relates generally to electroless gold plating methods and solutions, and more particularly to electroless plating with acidic potassium gold cyanide solutions free of extrinsic chemical reducing agents.

In semiconductor fabrication, gold-plated surfaces are extensively employed primarily to produce an ohmic contact with a semiconductor material, such as silicon or germanium. However, the gold-plated surface also serves to provide a controlled diffusion base for gold metal, to protect certain surfaces from chemical attack during acid etching, to protect surfaces from oxidation and corrosion, and to permit other metals to be easily soldered thereto. Depending on the type of semiconductor device, the gold may be plated directly on the semiconductor material, on the fabricated piece part, or on both.

Various methods including vacuum deposition, electrodeposition, immersion, and electroless deposition are methods commonly employed in gold plating. However, one major difliculty found with the use of such methods is the limitation of the thickness deposit. Another difiiculty is the procurement of a suitable adhesion of the gold plate to the base material. Other difliculties generally encountered are low deposition rates, inconsistent porosity of the gold deposit, and the inability to use all the gold in the plating bath, thereby resulting in high cost.

Accordingly, the object of this invention is an improved method of gold plating metallic surfaces.

Another object of this invention is an improved immersion method of gold plating to permit an increased thickness deposit on the base material.

Another object of this invention is an improved immersion method of gold plating semiconductor surfaces to improve the adhesive strength of the gold plate to the base material.

The novel features which are believed to be characteristic of the present invention, together with further objects and advantages thereof, will be understood from the following description in which the invention is illustrated by way of example.

The term immersion plating as utilized herein is the reduction of the gold ions in the plating solution to the metallic state through electrical energy produced by the potential difference which exists between the metal of the base material to be plated and said gold ions. It is to be expressly understood that no external source of electricity is employed and thus no electrode (anode) is required.

Heretofore, a generally known type of immersion plating formulation utilized was that having a potassium gold cyanide bath wherein either potassium hydroxide was added to produce a high pH or carbonates were added to produce a moderate alkaline pH, such as that disclosed in Patent 2,976,181, issued to R. R. Brookshire, March 21, 1961. I have discovered, however, that the usage of a potassium gold cyanide bath having an acidic pH ranging from 2.0 to 3.5 results not only in a superior adhesion of the gold plate to the base material but also in an increased thickness deposit with substantially all the gold from the bath being plated.

In general, the solution or bath for plating or coating the base material contains potassium gold cyanide of approximately 67 percent gold content (12.5 grams/ liter) in approximately 1000 ml. of distilled or deionized water.

A soluble acid or acid salt agent forming one of the basic components of the electrolyte is added to the bath, said acid agent being either a strong or a weak acid or a combination of acids or acid salts, such agents including hydrofluoric acid, hydrochloric acid, nitric acid, sulfuric acid, ammonium fluoride, and acetic acid. The pH of the solution is of a value in the range of 2.0 to 3.5 and is adjusted to a prescribed value by the addition of ammonium hydroxide. The temperature of the solution is maintained between 70 C. and 100 C. and the time required for plating most base materials ranges from about 10-30 minutes. The deposition rate varies with the acid agent used, the pH, and the temperature; for example, the deposition rate may be increased by raising the temperature or by adding a particular acid to lower the pH. However, such a deposition rate increase may affect the adhesion of the deposit.

Examples of base semiconductor materials, metals and alloys which have been coated or plated with gold by the practice of the present invention include silicon, germanium, Kovar, copper, nickel, electroless nickel, solder, silver, titanium, chromium, brass, stainless steel, tantalum and gold. It is to be understood that the term base metallic surface" used herein and in the claims includes any of the metals, semiconductor materials and alloys hereinbefore mentioned.

It is apparent that the values stated in the ranges set forth above in the general plate formulation will vary according to the particular type base material plated, since there are different requirements for the plated base materials. For example, in the semiconductor art it is required that an electrical contact be made to the plated base material, the type of contact utilized varying widely and therefore necessitating the usage of a different type plated surface. The following are illustrative examples of plating bath compositions for certain base materials, the conditions and ranges being in accordance with the present invention and the quantities, where applicable, being expressed in terms of grams per liter.

EXAMPLE I Gold plating of germanium Potassium gold cyanide 12.5 grams/liter (67% gold content). Deionized water To required volume. Sulfuric acid pH 2.5-3.0. Temperature -85 C.

EXAMPLE II Gold plating of silicon Potassium gold cyanide 12.5 grams/liter (67% gold content). Deionized water As required. Sulfuric acid pH 3.03.5. Hydrofluoric acid pH 2.53.0. Temperature Start at 70 C. and increase to 80-85 C. after initial deposit has formed.

It is believed that the sulfuric acid stabilizes the bath while the hydrofluoric acid used in Example II activates the silicon surface.

It is further believed that the gold in each formula is deposited from the potassium gold cyanide molecule and not from an acid salt of gold such as gold sulphate, gold fluoride, or gold chloride, as evidenced by the fact that said acid salt decomposes in hot water and therefore could not exist in the heated plate bath described in this invention.

It has been found that the above-described plating formulations are capable of depositing effectively percent of the gold contained in the bath. An experiment was conducted to determine the precise quantity of gold that could be plated from a typical acidic bath while yet producing sound gold deposits. Accordingly, a fifty ml. immersion gold bath containing 418.7 mg. of gold was acidified with sulfuric acid to a pH of 2.5 and operated at 8085 C. The results disclosed that 42.5 square inches of germanium had a deposit of 204.8 mg. of gold, the average thickness of the gold being 15 microinches. From the same bath 28.5 square inches of Kovar was gold plated with 193.2 mg. of gold deposit having an average thickness of 21 microinches. Thus, 398 mg. of gold was successfully plated from a gold bath having 418.7 mg. of gold therein, a balance of 20.7 mg. of gold remaining in the bath. However, it is to be especially noted that the balance of 20.7 mg. of gold was found to be drag-out losses and that a spectographic analysis of the spent solution showed that only 0.02 mg. of gold actually remained in the bath. It is also noted that a uniform, adherent deposit resulted on each base material utilized and that further experimentation resulted in the achievement of bulk handling.

The purity of the deposited gold is quite important in producing satisfactory ohmic contact to the gold plated silicon or germanium wafer of a semiconductor. In certain cases antimony must be present in the gold to produce a degenerate contact on N-type semiconductor- When the antimony gold plate is used to create a degenerate ohmic contact, it is desirable to sinter the antimony gold into the germanium base material not for adhesion but to acquire optimum contact characteristics. The bath life at a pH of 2.5 varies greatly with the acid used to suppress the pH. Bath life may be as short as -15 minutes or up to many hours, depending on the acids used. It is noted, however, that hath life does not indicate the useful life of the bath since the precipitate may be redissolved by raising the pH and the bath used to exhaustion of the gold solution.

It is to be understood that the above-described arrangements are simply illustrative of the application of the principles of the invention. Numerous other arrangements may be readily devised by those skilled in the art which will embody the principles of the invention and fall within the spirit and scope thereof.

What is claimed is:

1. An electroless method of gold plating a metallic surface, which comprises contacting the surface to be plated with a solution consisting essentially of an aqueous solution of potassium gold cyanide, the gold content of the potassium gold cyanide being at least 67% by weight, and a sufficient quantity of an acid to set the pH of the solution at a value between 2.0 and 3.5, the solution being free of extrinsic chemical reducing agents such as hypophosphite and hydrazine compositions.

2. The method as recited in claim 1, wherein the temperature of the solution during plating is between and 100 C.

3. The method as recited in claim 1, wherein the solution is acidified with a mineral acid selected from the group consisting of sulfuric acid, hydrochloric acid, nitric acid and mixtures of any of the foregoing with a fluoride selected from the group consisting of ammonium fluoride and hydrofluoric acid.

4. An electroless method of gold plating a germanium surface, which comprises contacting the surface to be plated with a plating solution consisting essentially of an aqueous solution of potassium gold cyanide, the gold content of the potassium gold cyanide being at least 67% by weight, and a sufiicient quantity of sulfuric acid to set the pH of the solution at a value between 2.5 and 3.0, the solution being free of extrinsic chemical reducing agents such as hypophosphite and hydrazine compositions, the solution being heated to a temperature between and C.

5. The method as recited in claim 4, wherein the solution additionally contains approximately 0.25 gram per liter of potassium antimonial tartrate.

6. An electroless method of gold plating a silicon surface, which comprises contacting the surface to be plated with a plating solution consisting essentially of an aqueous solution of potassium gold cyanide, the gold content of the potassium gold cyanide being at least 67% by weight, a sufficient quantity of sulfuric acid to set the pH of the solution at a value between 3.0 and 3.5, and a sufiicient quantity of hydrofluoric acid to activate the silicon surface and to set the pH at a final value of 2.5 to 3.0, the solution being free of extrinsic chemical reducing agents such as hypophosphite and hydrazine compositions, the solution being initially heated to a temperature of about 70 C. and being further heated after an initial gold deposit has formed to a temperature between 80 and 85 C.

References Cited by the Examiner UNITED STATES PATENTS 2,814,589 11/57 Waltz 117-130 X 2,836,515 5/58 McNally 117-130 2,937,962 5/60 Kitchens et al 148-333 2,983,854 5/61 Pearson 148-333 3,013,955 12/61 Roberts 117-130 X 3,032,436 5/62 Gostin et al 117-130 OTHER REFERENCES Lee: Corrosion Technology, March 1963, TA 462 C68, p. 68.

Parker: Plating, June 1959, TS 670 A3, pp. 622-623.

RICHARD D. NEVIUS, Primary Examiner. WILLI M D- N, Examiner. 

1. AN ELECTROLESS METHOD OF GOLD PLATING A METALLIC SURFACE, WHICH COMPRISES CONTACTING THE SURFACE TO BE PLATED WITH A SOLUTION CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF POTASSIUM GOLD CYANIDE, THE GOLD CONTENT OF THE POTASSIUM GOLD CYANIDE BEING AT LEAST 67% BY WEIGHT, AND A SUFFICIENT QUANTITY OF AN ACID TO SET THE PH OF THE SOLUTION AT A VALUE BETWEEN 2.2 AND 3.5, THE SOLUTION BEING FREE OF EXTRINSIC CHEMICAL REDUCING AGENTS SUCH AS HYPOPHOSPHITE AND HYDRAZINE COMPOSITIONS. 